1. Field of the Invention
The present invention relates to a meteorological radar apparatus used for the observation of meteorological phenomena such as cloud, rain and fog.
2. Description of the Prior Art
A Doppler radar apparatus which enables the close observation of time and spatial changes in the wind has been recently used as means for observing meteorological phenomena such as cloud, rain and fog.
Generally speaking, a Doppler radar apparatus for meteorological observation (to be referred to as "meteorological radar apparatus" hereinafter) projects a pulse wave (to be referred to as "transmission pulse signal" hereinafter) consisting of a plurality of pulses onto a target such as cloud, rain or fog which is an object of observation, measures a difference in Doppler phase between received pulses based on a Doppler effect from a pulse wave (to be referred to as "received pulse signal" hereinafter) reflected from the target, and calculates the Doppler velocity of the target based on this difference in Doppler phase. There are signal processing systems for calculating the Doppler velocity of the target: a FFT system in which a frequency spectrum is obtained by sampling each of the pulses of reflected received pulse signals and converting a time series of the received pulse signals with FFT processing, and a pulse pair processing system in which an average phase difference between received pulse signals is obtained based on a phase change between two pulses.
As a transmitter for this type of meteorological radar apparatus, a magnetron transmitter using a magnetron transmission tube (self-oscillation transmission tube) is used to meet such requirements as easy production and low costs.
For example, an MTI radar apparatus using a magnetron as a transmitter is described in Japanese Laid-open Patent Application No. Hei 3-54495. Part of transmission signals from a magnetron 1 are taken out by a directional coupler 17 to form a phase lead-in signal as a reference for the phase detection of phase detectors 1 to 4 or 11 and 12, and the phase of a reception signal received by an antenna 4 is detected based on this phase lead-in signal to obtain the output of a highly stable and highly accurate phase detected wave.
Generally speaking, in a Doppler radar apparatus of the prior art using a self-oscillation transmitter such as a magnetron (a magnetron transmitter will be described hereinunder as an example), there are various Doppler velocity measurement errors based on the frequency instability of this magnetron. To compensate for the Doppler velocity measurement errors caused by the instability of the frequency characteristics of this magnetron, various systems are employed. In a radar apparatus disclosed by the above Japanese Laid-open Patent Application No. Hei 3-54495, a phase lead-in signal is formed from a transmission pulse signal output from a magnetron for each transmission pulse and the phase of a received pulse signal is detected based on this phase lead-in signal to prevent deterioration in measurement accuracy by compensating for a difference in initial phase between transmission pulses, which is one of the causes of the Doppler velocity measurement errors caused by the instability of the frequency of the magnetron.
In contrast to this analog phase compensation system (to be referred to as "analog phase lock system" hereinafter), there is a system for correcting the phase of a received pulse using a converted digital signal. For example, a system (to be referred to as "digital phase lock system" hereinafter) in which the phase of a reception signal is digitally corrected using a converted digital signal is disclosed in the chapter of "B. Amplitude and Phase Memory" on page 283, left column of "The RONSARD Radars: A Versatile C-band Dual Doppler Facility", IEEE TRANSACTIONS ON GEOSCIENCE ELECTRONICS, Vol. GE-17, No. 4, October 1979. A meteorological radar apparatus employing this digital phase lock system does not need to adjust the phase of a signal output from a COHO (Coherent Oscillator) directly and can correct the phase of the signal digitally unlike the analog phase lock system. Therefore, sufficiently high phase correction accuracy can be obtained compared with the analog system.
According to radar apparatuses employing these reception systems, even when a magnetron transmitter having very instable frequency characteristics is used as a transmission unit, the phase measurement reference of a received pulse signal is set for each received pulse and deterioration in the measurement accuracy of the Doppler velocity caused by a difference in initial phase between transmission pulses can be prevented.
However, in the radar apparatuses of the prior art using the above-described reception systems (including a meteorological radar apparatus), although deterioration in the measurement accuracy of the Doppler velocity caused by a difference in initial phase between transmission pulses can be prevented, (1) a difference in output timing between transmission pulses and (2) deterioration in the measurement accuracy of the Doppler velocity caused by jitter or the like at the time of sampling a transmission waveform cannot be prevented. Even if the Doppler velocity of the target is observed by compensating for the difference of initial phase, the high-accuracy measurement of the Doppler velocity which is free from deterioration in the measurement accuracy of the Doppler velocity based on the frequency characteristics of the magnetron transmitter cannot be realized.
Deterioration in the measurement accuracy of the Doppler speed caused by a difference in output timing between transmission pulses and deterioration in the measurement accuracy of the Doppler velocity caused by jitter or the like at the time of sampling a transmission waveform will be described with reference to FIG. 15 and FIG. 16, respectively. FIG. 15 is a diagram showing the output relationship between transmission pulses output from the magnetron transmitter and a master trigger and FIG. 16 is a diagram showing the pulse characteristics of each transmission pulse shown in FIG. 15. FIG. 15 shows the output relationship between two arbitrary first and second transmission pulses of a transmission signal consisting of a plurality of pulses and a master trigger. The term "master trigger" is a synchronizing signal which is the basis for time synchronization between transmission operation and reception operation and a signal for specifying the pulse repetition frequency of a transmission pulse signal projected onto an object of observation.
When the frequency characteristics of the transmitter are stable, the pulse repetition cycle of a transmission pulse signal from a transmission antenna becomes constant according to the pulse repetition frequency. However, in the case of a radar apparatus using a self-oscillation transmitter such as a magnetron transmitter, the frequency characteristics of the transmitter are very instable and each transmission pulse of a transmission pulse signal is output from the transmitter before or after the master trigger which is a synchronization signal. This transmission operation is an operation based on the instability of the frequency of the transmitter which can occur even when the trigger pulse cycle of a trigger signal to be applied to the transmitter is set constant according to the pulse repetition frequency. The relationship between the output timing of the master trigger and the output timing of a transmission pulse is such as shown in FIG. 15 (in FIG. 15, the first transmission pulse is synchronized with the master trigger but the second transmission pulse is not synchronized with the master trigger). The transmission pulse signal consists of a plurality of pulses. Thus, the transmission pulse signal output from the magnetron transmitter having instable frequency characteristics such as a magnetron transmitter includes a transmission pulse which is output before or after the master trigger. The transmission pulse signal having an irregular pulse repetition cycle as a whole is projected onto the object of observation.
A received pulse signal reflected from the object of observation is sampled by an A/D converter according to the output timing of the master trigger which is a synchronization signal as described above. The sampling positions of received pulses sampled by the A/D converter are the same because the transmission timing of a transmission pulse signal is output in synchronism with the output timing of the master trigger. As described above, as for a received pulse signal corresponding to a transmission pulse signal which is asynchronous with the master trigger, that is, output before or after the master trigger, the sampling positions of received pulses differ from each other.
The measurement of the Doppler velocity is carried out based on a difference in Doppler phase between received pulses measured at the sampling positions, that is, a difference in Doppler phase between received pulses. When the sampling positions of received pulses differ from each other due to a difference of transmission timing as described above, the Doppler velocity of the object of observation is observed from each of the differences in Doppler phase measured at the sampling positions which differ from each other. For example, as for received pulses (unshown) corresponding to first and second transmission pulses shown in FIG. 15, the Doppler phase of a first received pulse is measured at a position "a" which is a rising portion of the pulse and the Doppler phase of a second received pulse is measured at a position "b" which is a falling portion of the pulse (provided that sampling is carried out upon a rise in the master trigger).
The pulse characteristics of a transmission pulse signal output from a magnetron transmitter as shown in FIG. 16 will be described and deterioration in the measurement accuracy of the Doppler velocity based on the pulse characteristics of this transmission pulse signal will be detailed hereinunder. The pulse characteristics of a transmission pulse signal output from the magnetron transmitter have time-amplitude characteristics and time-phase characteristics as shown in FIG. 16 due to the instability of its frequency characteristics. In FIG. 16, an upper graph shows amplitude characteristics and a lower graph shows phase characteristics. Time is plotted on the axes of abscissa of the upper and lower diagrams. The phase of a transmission pulse output from the magnetron transmitter changes in a complex shape (phase change rate is not constant) from a rise to a fall in pulse as shown in the lower phase characteristic diagram of FIG. 16.
Therefore, even when the Doppler velocity of a stationary object is measured from a received pulse signal reflected from the stationary object, if the sampling positions of received pulses differ from each other as shown in FIG. 15, the Doppler velocity of the stationary object is measured from a difference in Doppler phase between received pulses measured at sampling positions which differ from each other.
For instance, when received pulse corresponding to the transmission pulses shown in FIG. 15 are reflected from the stationary object and the phase measurement position of a first received pulse is a pulse rising portion (position indicated by a left arrow in the amplitude characteristic diagram) and the phase measurement position of a second received pulse is a pulse rising portion as shown in FIG. 16 (position indicated by a right arrow in the amplitude characteristic diagram), the object of observation which has actually a Doppler velocity of 0 is judged to have a Doppler phase difference Ti as shown in the lower phase characteristic diagram of FIG. 16 and the object of observation is considered to move at a Doppler velocity corresponding to the Doppler phase difference Ti.
Generally speaking, when the Doppler velocity of the object of observation is calculated by a pulse pair processing method or the like, the Doppler velocity of the object of observation is calculated from a difference in Doppler phase between two arbitrary received pulses of a received pulse signal reflected from the object of observation. When the Doppler phases of these received pulses are measured at the same phase measurement position, the Doppler velocity of the object of observation which does not move by itself, such as a building, does not produce a Doppler effect, the difference in Doppler phase between the received pulses is measured to be zero, and the Doppler velocity is observed as zero. However, a transmission pulse signal from the magnetron transmitter is output at a transmission timing different from the output timing of the master trigger as described above. Therefore, when a received pulse signal corresponding to the transmission pulse signal output at this transmission timing is sampled at the output timing of the master trigger, the sampling positions of received pulses, that is, the phase measurement positions of received pulses differ from each other, and an erroneous Doppler velocity is observed.
In the radar apparatus using the magnetron transmitter, the transmission timing of a transmission pulse signal from the transmitter differs from the output timing of the master trigger due to the instability of the characteristics of the magnetron. Even if a difference in initial phase between the transmission pulses of a transmission pulse signal are compensated, an error is produced in the measurement of the Doppler phase of each received pulse due to the above-described difference of transmission timing and a phase change between transmission pulses, whereby the measurement accuracy of the Doppler velocity greatly lowers. The measurement error of the Doppler velocity due to the difference in transmission timing of a transmission pulse signal is called "bias error of Doppler velocity" to discriminate it from an error caused by the above-described difference in initial phase between transmission pulses. The pulse characteristics of a transmission pulse differ according to the type or the like of a magnetron used in each transmitter. In an amplifying tube such as a klystron, there is no phase change between transmission pulses as shown in the phase characteristic diagram of FIG. 16, that is, the phase is constant from a pulse rise time to a pulse fall time. Even if the phase measurement positions of the received pulses of a corresponding received pulse signal differ from each other, the measured Doppler phases are almost the same and the above-described problem of the magnetron transmitter hardly arises.
Another measurement error of the Doppler velocity produced based on a phase change between transmission pulses shown in FIG. 16 is a random error of the Doppler velocity. This is an error produced by jitter at the time of sampling a received pulse signal unlike a measurement error of the Doppler velocity produced based on a difference in transmission timing of a transmission pulse signal. This occurs when the sampling timing of a received pulse is shifted from a predetermined sampling position by the jitter of the A/D converter. For example, this is an error produced by a shift of the sampling position within the range shown by slant lines in FIG. 16.
Therefore, to measure a highly accurate Doppler velocity by preventing deterioration in the measurement accuracy of the Doppler velocity caused by the instability of the characteristics of the magnetron transmitter, the Doppler velocity must be measured without deteriorating the measurement accuracy of the Doppler velocity caused by measurement errors in consideration of the above-described two errors (bias error and random error).
A meteorological radar apparatus must be able to receive reflected waves by rain drops frequently and measure a rain cloud at a certain measure of distance through a layer of rain. In many cases, a C band wavelength (.lambda.=5 cm) is used. To measure cloud particles having a particle diameter smaller than several tens of micrometers and fog, a pulse wave having a wavelength shorter than a centimetric wave, for example, an electromagnetic wave having a frequency band such as a W band (.lambda.=3 mm) or Ka band (8.7 mm), must be used. When the Doppler velocity of the target is measured using a high-frequency electromagnetic wave having a relatively short wavelength, the pulse interval of an electromagnetic wave projected onto the target, that is, the pulse repetition cycle must be set to a time shorter than that of the C band from its relation with the aliasing of the Doppler velocity (measurable maximum speed range), that is, signal reproducibility.
Generally speaking, the higher the use frequency band the higher the Doppler frequency becomes. To grasp the contents of a reception signal having a high Doppler frequency accurately, the reception signal must be sampled a large number of times. To increase the number of sampling times, the pulse repetition cycle (pulse interval) must be made short, whereby the influence of an multiple-trip echo becomes large. That is, when the Doppler velocity of the target is measured by a transmission pulse signal having a relatively short interval between transmission pulses, namely, pulse repetition cycle, a multiple-trip echo such as a second-trip echo, third-trip echo or fourth-trip echo reflected from an object other than the target is readily included into a received pulse signal reflected from the target and the Doppler velocity of the target must be calculated from a bad received pulse signal which is greatly influenced by the multiple-trip echo.
Although the conventional meteorological radar apparatus using a magnetron transmitter has such an advantage that the transmitter can be produced more easily and at a lower cost than a transmitter with an amplifying tube such as a klystron, the frequency characteristics of the transmitter are very instable, and measurement errors of the Doppler velocity are produced based on a shift of the transmission timing of a transmission pulse signal and jitter at the time of sampling a received pulse signal in addition to a difference in initial phase between transmission pulses, thereby reducing the measurement accuracy of the Doppler velocity of the target.
To measure cloud particles having a particle diameter smaller than several tens of micrometers and fog, a pulse wave whose wavelength is shorter than a centimeter must be used. In this case, the pulse repetition cycle of a transmission pulse signal must be set to a time much shorter than that of the C band from its relation with the aliasing of the Doppler velocity or the like, and the Doppler velocity of the target must be measured from a bad reception signal which is greatly influenced by a multiple-trip echo.